Field of the Invention
The present invention relates generally to multi-piece golf balls having a solid core and multi-layered cover. In one embodiment, the multi-layered cover is a two-layer cover including inner and outer cover layers. In another preferred embodiment, there is a three-layer cover including inner, intermediate, and outer cover layers. At least one of the cover layers comprises a thermoplastic composition and preferably the composition comprises an ethylene acid copolymer ionomer and plasticizer.
Brief Review of the Related Art
Multi-layered, solid golf balls are used today by recreational and professional golfers. In general, these golf balls contain an inner core protected by a cover. The core acts as the primary engine for the ball and the cover protects the core and helps provide the ball with durability and wear-resistance. The core and cover may be single or multi-layered. For example, three-piece golf balls having an inner core, inner cover layer, and outer cover layer are popular. In other instances, golfers will use a four-piece ball containing a dual-core (inner core and surrounding outer-core layer) and dual-cover (inner cover layer and surrounding outer cover layer). Intermediate layer(s) may be disposed between the core and cover layers to impart various properties. Thus, five-piece and even six-piece balls can be made. Normally, the core layers are made of a natural or synthetic rubber material or an ionomer polymer. These ionomer polymers are typically copolymers of ethylene and methacrylic acid or acrylic acid that are partially or fully neutralized. In particular, highly neutralized polymer (HNP) compositions may be used to form a core layer. Metal ions such as sodium, lithium, zinc, and magnesium are commonly used to neutralize the acid groups in the copolymer.
Such ethylene acid copolymer ionomer resins generally have good durability and toughness. These ionomers may be used to make cover, intermediate, and core layers for the golf ball. When used as a core material, the ionomer resin helps impart a higher initial velocity to the golf ball. When used as a cover material, the ionomer resin helps impart impact durability, wear-resistance, and cut/shear-resistance to the golf ball.
Many golf balls used today have multi-layered covers comprising an inner cover and surrounding outer cover layer. For example, the inner cover may be made of a relatively hard material such as the above-described ethylene acid copolymer ionomer resin. Meanwhile, the outer cover layer may be made of a relatively soft polyurethane or polyurethane composition. Also, three-layered covers, wherein the cover structure includes an inner cover, intermediate cover, and outer cover layer may be made. For example, Sullivan, US Patent Application Publication 2002/0151380 discloses a golf ball having a core and cover wherein the cover comprises: a) an inner cover layer having a first thickness and being disposed directly about the core; b) an outer cover layer having a second thickness no greater than about 0.050 inches; and c) an intermediate cover layer having a third thickness and being disposed between the inner and outer cover layers; wherein the outer cover layer comprises a composition formed of a reactive liquid material (thermoset material comprising polyurethane, polyurea, polyurethane ionomer, epoxy, or a mixture thereof) and the combination of the first, second, and third thickness is no greater than about 0.125 inches.
Sullivan et al., US Patent Application Publication 2004/0235587 discloses a golf ball having a core and a cover comprising: an inner cover layer; an outer cover layer having a material hardness of 60 Shore D or less; and an intermediate cover layer disposed between the inner and outer cover layers. At least two of the inner, intermediate, and outer cover layers comprise a non-ionomeric material. Preferably, the outer cover layer comprises a polyurethane, a polyurea, a copolymer of a polyurethane, a copolymer of a polyurea, or an interpenetrating polymer network.
Although some ethylene acid copolymer ionomer compositions may be somewhat effective for making certain components and layers in a golf ball, there is still a need for new compositions that can impart high performance properties to the ball. Particularly, there is a continuing need for improved cover constructions in golf balls. The cover material should have good impact durability, toughness, and cut/shear-resistance, while helping to provide the ball with high resiliency. The cover material, however, should not be excessively hard and stiff so that properties such as feel, softness, and spin control are sacrificed. The present invention provides golf balls having an optimum combination of properties.